A need for a wireless electrical power supply has spurred an interest in piezoelectric energy harvesting on the extraction of electrical energy using a vibrating piezoelectric device according to G. K. Ottman et al. in an article entitled “Adaptive Piezoelectric Energy Harvesting Circuit for Wireless Remote Power Supply,” IEEE Transaction on Power Electronics, Vol. 17, No. 5, September 2002. As stated therein, a vibrating piezoelectric device differs from a typical electric power supply in that its internal impedance is capacitive rather than inductive, and that it may be driven by mechanical vibration of varying amplitude and frequency.
Centimeter-scale piezoelectric elements generate milliwatt-range electric power using ambient vibration below 1 kHz. Such elements have recently been considered as a viable solution for long life micropower generators since they generate sufficient power to drive low-power electronic devices such as smart wireless sensors that consume less than a few milliwatts, as suggested by A. Tabesh et al. in an article entitled “A Low-Power Stand-Alone Adaptive Circuit for Harvesting Energy From a Piezoelectric Micropower Generator,” published by the IEEE Transaction on Industrial Electronics, Vol. 57, No. 3, pp. 840-849, March 2010.
The aforementioned A. Tabesh et al. article discloses an adaptive energy-harvesting circuit with low power dissipation that is useful for efficient AC/DC voltage conversion of a piezoelectric micropower generator. The circuit operates stand-alone and extracts the piezoelectric strain energy independent of the load and piezoelectric parameters without using any external sensor. The circuit consists of a voltage-doubler rectifier, a step-down switching converter, and an analog controller operating with single supply voltage in the range of 2.5-15 volts. The controller uses the piezoelectric voltage as a feedback and regulates the rectified voltage to adaptively improve the extracted power. The nonscalable power dissipation of the controller unit is less than 0.05 mW, and the efficiency of the circuit is about 60% for output power levels above 0.5 mW. Experimental verifications of the circuit show the following: 1) the circuit notably increases the extracted power from a piezoelectric element compared to a simple full-bridge diode rectifier without control circuitry, and 2) the efficiency of the circuit is dominantly determined by its switching converter. The simplicity of the circuit facilitates the development of efficient piezoelectric energy harvesters for low-power applications such as wireless sensors and portable devices.
The concept of an autonomous electric device is no longer the subject of imagination according to Liya Ghu and Renwen Chen in an article entitled, “A New Synchronized Switch Harvesting Scheme Employing Current Doubler Rectifier,” published by Sensors and Actuators, Vol. 174 (2012) 107-114. As disclosed therein, a piezoelectric energy harvester consists of three parts: a piezoelectric energy harvester structure working under vibration excitation, an electrical interface that converts the generated AC current into DC current compatible with most terminal electric loads and an energy storage device that accumulates and stores energy for intermittent use.
A U.S. patent of Campbell, U.S. Pat. No. 8,269,399 B2, discloses a system and apparatus for harvesting energy. This patent discloses a sensor assembly including a transducer and a control module coupled with the transducer. The control module is configured to selectively switch the sensor assembly between a first mode of operation wherein the sensor assembly measures an amount of energy induced to the sensor assembly, and a second mode of operation wherein the sensor assembly stores an amount of energy induced to the sensor assembly.
A U.S. patent of Lee et al., U.S. Pat. No. 8,373,332 B2 issued on Feb. 12, 2013 discloses an energy harvesting electric device that is capable of increasing output power. The energy harvesting electric device includes an energy harvesting array having a plurality of energy harvesters, a single rectifier connected to the energy harvester array, and an output unit which is connected to the single rectifier and has a load resistance. The energy harvesters include a plurality of first energy harvesters connected to each other in parallel and a single second energy harvester connected in parallel to the first energy harvesters. The first energy harvesters have a first specific resistance higher than the load resistance and the second energy harvester has a second specific resistance higher than the first specific resistance.
Piezoelectric energy harvesters have been used to feed loads in military, non-military and many other applications. Many techniques have been used to extract the maximum power from piezoelectric materials. For example, a considerable amount of power is dissipated in the piezoelectric materials to flip the output voltage through an internal capacitor and resistor of a piezoelectric device. One approach to harvest this power is the synchronized switch harvesting on inductor technique. Other circuits use the accumulated power in a battery to feed the circuit to detect the flipping points of piezoelectric harvesters.
The output power from piezoelectric energy harvesting (PEH) is an AC current with amplitude and frequency depending on the mechanical stress on a piezoelectric material and the electric circuit connected to it. The equivalent circuit for a piezoelectric device PZT is a current source in shunt with a resistor and a capacitor as shown in FIG. 1. The power generated from a piezoelectric energy harvester is very small to be used to feed loads directly and it is not suitable to be used in most of the applications without battery storage or super-capacitor to accumulate the generated power to be used when required. For this reason, a rectifier should be used to convert AC to DC power.
The literature recommended using a diode bridge rectifier [1] rather than half-wave diode rectifier. Also, a voltage multiplier can be used to increase the dc output power extracted from PEH [2]. Voltage multipliers increase the output voltage from a piezoelectric energy harvester and extend the operating range of piezoelectric energy harvester [3]. The DC output power from a rectifier may be used directly to feed a load or to charge a storage device as for example a Standard PEH, SPEH [4] as shown in FIG. 1. This technique is characterized by high output voltage and low current which is not suitable for small battery storage with low voltage. Operating a PEH circuit in low voltage will considerably reduce the efficiency and the extracted power. Therefore, a device that can interface a battery having low voltage with high output voltage from a piezoelectric device is required.
Ottman and et. al. [5] and [6] introduced the first DC-DC buck converter to interface the low voltage battery with high voltage output from a piezoelectric device. Ottman and et. al. claim that the power extracted from a piezoelectric device is increased by 400% in [5] and 325% in [6] and [7] as compared to when the battery was directly charged with a piezoelectric element-rectifier circuit without a DC-DC converter (SPEH of FIG. 1). Due to the step down only characteristics of a buck converter other articles introduce a buck-boost converter to extend the operating range of the converter to work as a step-down or step-up [8], [9].
Fly-hack converters discussed in some articles extend the operating limits of the piezoelectric energy harvester [10]. In most cases the DC-DC converter is a must because it extends the operating range, increases the power extracted from a piezoelectric energy harvesting circuit, and considerably increases piezoelectric energy harvesting efficiency. The transition of the piezoelectric device output voltage between its peaks happens when the load is isolated from the piezoelectric device (all rectifier diodes are in OFF state), as shown in FIG. 2, which dissipates a considerable amount of energy through an internal capacitor and resistor of the piezoelectric device. This considerably reduces the extracted energy and efficiency of the piezoelectric energy harvesting circuit. Flipping the voltage through an inductor at short time can extract most of this dissipated power to the load and increases the output power considerably. This technique is known as Synchronized Switch Harvesting on Inductor (SSHI). This technique was first introduced by [11]. The authors claim that the technique exhibits an output power increase by over 900% compared to the same piezoelectric system with SPEH that shown in FIG. 1 [11].
Some researchers used inductor in parallel with PZT [11] [12], [13], [14], [15], [16] and others used the inductor in series [17], [18]. Depending on the results and comparison of literatures it has been concluded that the parallel SSHI technique is considered one of the best techniques that have been used to flip the output voltage of the piezoelectric device. This is the reason it has been used in this research. Two different techniques can be used for driving the parallel SSHI circuit. One is called self-power SSHI (SP-SST-II) [12], [19], [20], [21] in which the generated power feeds the auxiliary circuits to detect the correct points to flip the terminal voltage of the piezoelectric energy harvesting circuit. Some other researchers used accumulated energy in the battery (BD-SSHI) to feed the driving circuit [22], [23] of parallel SSHI. BD-SSHI used the battery power or any auxiliary circuit to feed the integrated circuits that used to detect the peaks and to drive the SSHI circuit. BD-SSHI technique suffers from low efficiency and it is not self-started when the battery loses its charge. SP-SSHI technique used the peak detector circuit to detect the time to switch ON or OFF the switches to flip the voltage across a piezoelectric device through the inductor L and Cp [24]. This circuit is accomplished by using a differentiating circuitry that senses the slope of the capacitor's voltage (dv/dt), and a comparator to switch two metal oxide-semiconductor field effect transistors (MOSFETs) in series with the SSHI inductor. The differentiating circuit has been accomplished by RC circuit that detects the absolute peaks of the piezoelectric device terminal voltage when it starts to change its direction. The signal should be supplied to the comparator to switch MOSFETs. The main idea here is the biasing voltage can be obtained by charging two capacitors through half-wave rectifiers. This circuit [24] dissipated a considerable amount of power in feeding and biasing the comparator and other elements of the circuit.
The other peak detector circuit is shown FIG. 3. The flipping technique used in this circuit has been first introduced by [25]. This technique has been used widely in many researches such as [20]. This circuit used peak detector circuits having 8 diodes, 4 transistors, three capacitors, and two resistors. In this circuit, two voltage inversions and one charge neutralization are experienced for exerting each switching action. In case of switching on positive peak voltage, R1, D1 and C1 form an envelope detector. T3, D3 and T1 will be switched ON and the current will resonate through SSHI inductor and D5 and D8. In the same way, switching on negative peak voltage, R2, D2 and C2 form an envelope detector. T4, D4 and T2 will switch ON and the current will resonate through SSHI inductor and D6 and D7. The load can be connected across the capacitor C or it can be connected after using diode bridge rectifier and DC-DC converter. The current and voltage waveforms of piezoelectric energy harvesting circuit using parallel SSHI technique is shown in FIG. 4.
Notwithstanding the above, it is presently believed that there is a need and a potential commercial market for a novel piezoelectric energy harvester using a self-power SSHI circuit as disclosed in the present invention. There should be a demand and a commercial market for such devices because such devices use passive elements, i.e. two capacitors, to detect the flipping points of terminal voltage, rather than the many active elements such as resistors, diodes and op-amps that are used in conventional circuits. Further, Applicants' self-powered synchronized switch harvesting on inductor circuit showed a significant reduction in losses, increased output power, increased efficiency, low costs and increased reliability.